class Solution {
public:
    int dx[4] = { 0, 0, 1, -1 };
    int dy[4] = { 1, -1, 0, 0 };

    int maxDistance(vector<vector<int>>& grid) {
        int m = grid.size(), n = grid[0].size();
        queue<pair<int, int>> q;
        vector<vector<int>> h(m, vector<int>(n, -1));
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1)
                {
                    q.push({ i,j });
                    h[i][j] = 0;
                }
            }
        if (q.size() == 0 || q.size() == m * n) return -1;

        int ret = -1;
        while (q.size())
        {
            int sz = q.size();
            while (sz--)
            {
                auto [a, b] = q.front();
                q.pop();
                for (int i = 0; i < 4; i++)
                {
                    int x = a + dx[i], y = b + dy[i];
                    if (x >= 0 && x < m && y >= 0 && y < n && h[x][y] == -1)
                    {
                        q.push({ x, y });
                        h[x][y] = h[a][b] + 1;
                        ret = max(ret, h[x][y]);
                    }
                }
            }
        }
        return ret;
    }
};